In a gas turbofan jet engine powering an aircraft in flight, certain environmental conditions cause ice formation on the rotating fan blades and adjacent rotating components during engine operation. When this ice sheds, it is thrown radially outwardly and aft by the fan blades due to centrifugal force and impinges against the annular outer liners facing radially inwardly from the fan case. Accordingly, the fan case liners in this region of the engine are specifically configured for having increased strength for better resisting impact from the ice.
Since the turbofan engine powers an aircraft in flight, the weight of the fan case liner is a significant factor affecting the overall weight of the engine and efficiency of operation of the aircraft. Accordingly, it is difficult to make an effective impact resistant fan case liner that is also lightweight. For the ever increasing size of modern aircraft turbofan engines with increasing diameter fans therein, the diameter of the fan case liner correspondingly increases which in turn increases its weight even further, with the weight thereof becoming even more significant in obtaining efficient operation of the aircraft.
Furthermore, the impact resistant liner is typically integrated into a conventional honeycomb sandwich construction, sound suppression liner which similarly attaches into the fan case that surrounds the fan. The impact resistant liner must therefore be structurally compatible with the sound suppression liner while still maintaining effective impact resistance with as little weight as possible.
In U.S. Pat. No. 5,160,248 by J. P. Clarke, entitled "Fan Case Liner for a Gas Turbine Engine With Improved Foreign Body Impact Resistance," assigned to the present assignee, several embodiments of an improved fan case liner having integrated portions for impact resistance and sound suppression are disclosed. The integrated liner includes a multi-ply facesheet supported by one or more honeycomb cores and surrounded by a backsheet. The facesheet includes multiple plies of fiberglass or the like impregnated with an epoxy resin and cured for forming an impact resistant material. The face sheet is joined to a honeycomb layer of aluminum, for example, by a fiber-reinforced adhesive. In the preferred embodiment disclosed by Clarke, a second honeycomb layer, also being aluminum for example, is attached to the first honeycomb layer and has a compressive strength greater than the compressive strength of the first honeycomb layer for improved distribution of foreign body impact forces and to resist plastic deformation of the liner. The higher the compressive strength of the honeycomb layer, the denser the honeycomb required, which increases weight of the liner.
Although the fan case liner disclosed by Clarke has improved impact resistance capability, the resulting facesheet itself is nevertheless subject to damage since it has a relatively low ductility epoxy matrix. Its combination with the supporting honeycomb layers, however, provides an effective composite article for accommodating impact loads from the foreign objects.